Display systems may employ a plurality of different formats. The video formats may include different aspect ratios. Video formats, with aspect ratios of e.g., 4:3, 16:9, 2:35:1, etc. are in use for broadcasting and/or for video storage mediums. The broadcast or video storage medium format is not necessarily the same as the display format. If no indication information (aspect ratio flags, e.g., wide screen signaling (WSS), etc.) is delivered with the video signal from the source, real-time analysis of the video content is necessary to find black bars if present.
While watching a program from a television (TV) station, many changes of the aspect ratio may occur. For example, advertisements may be broadcasted as 4:3 and/or as a letterbox picture, while a movie may be broadcasted as a letterbox picture.
Because of the nature of the live signal, typically letterbox detectors are only able to react after some time (normally 1-5 seconds) of analysis. As a result, switching from non-letterbox mode to letterbox mode and back always takes time and is delayed. All letterbox detectors which are based on active picture analysis are affected by this problem. The effect for the viewer is jumping/jittering pictures from time to time because this can happen from one frame to another within the same scene (e.g., width and/or height jumps and perhaps also centering jumps). This unpleasant effect occurs even more often if the detector has problems properly detecting the black bars during overall dark scenes or high complexity scenes (e.g., faulty detections/changes). In addition, all of these methods are more or less sensitive to the brightness, noise, the thickness of black bars on the sides or top of the picture and of course the overall complexity of the scenes.
State of the art letterbox detectors change their picture width and/or height as soon as they detect a change of the aspect ratio of the active video area to fill out the screen without displaying unwanted black bars. Some advanced detectors are even able to find logos and/or subtitle information. Such detectors shift the picture up or down together with expansion of the picture (for example, to cut out logos but, keep subtitles and re-center the picture). This change is not smooth; it is abrupt and therefore is disturbing to the viewer. It may especially be a problem when the detector is subjected to the conditions described above.
Letterbox detectors are widely used in the consumer electronics industry and are commonly implemented in mid-range to high-range color TVs (CTVs). Custom solutions are employed, which may include dedicated functions in well-known integrated circuits (ICs). The efficiency of these functions depends on the complexity of the picture analysis. Simpler stages analyze only the average brightness level for a certain number of lines at the top and bottom of the picture. More advanced stages also provide frequency analysis, noise analysis and are able to exclude some areas within the expected black bars to avoid logos from the broadcast station that can adulterate the result. Another technique is to include the detection of subtitles or news-flash information.
It is inevitable that faulty decisions are produced by these stages from time to time in current letterbox detectors. As a result, the CTV may change the displayed aspect ratio and/or resizes the picture back and forth without a change in the source. This unpleasant effect is clearly visible to the viewer, and represents a degeneration of the picture quality which is internally generated by the CTV set itself and should be at least minimized.